Soils aren’t just for farmers! Soils are fundamental to every ecosystem process we know—they are the foundation of our ecosystem. Soils provide physical support, store and cycle nutrients, regulate water quality, storage, and run-off, dictate the abundance and type of species present on the landscape, recycle organic matter, serve as filters for air, water, and organisms, and store carbon; they are also full of living creatures, bacteria, and fungi, housing innumerous species and millions of organisms. Whether you have a farm, forest, range, or just your own garden, soils are important to you.
Soil is comprised of four basic components: air (~25%), water (~25%), minerals (~45%) and organic material (~5%). The mineral component consists of sand, silt, and clay and the relative proportions of these minerals are what determine the texture, water-holding capacity, and productivity of the soil. You may be familiar with the ‘soil triangle’, which identifies different soil types based on their percent of each sand, silt, and clay. The most productive soils are those with a fairly even distribution of all three—called loam.
What makes different soils have different sand, silt, and clay ratios? Parent material is the basis of the mineral content. Many soils have more than one parent material. This is true in the Columbia Gorge, where ancient basalt flood-lava laid down layers hundreds of feet deep, followed by more recent volcanic lahars and, in a geologic blink of the eye, repeated scouring by the Missoula Floods. This parent material, along with organic inputs, climate, topography, biota, and time all contribute to the development of our soils.
Soil horizons are the parallel layers you notice in a cross-section of your ground. Horizons are identified beginning at surface level with the O horizon, and moving down through A, E, B and C horizons until finally arriving at the parent material, or bedrock. The topmost horizons have the most organic matter, as vegetation both above and below ground begin to decompose. The nutrients from the organic matter leach through the horizons with rainfall and other weathering agents, causing the color changes you can see.
Soil structure is another important consideration. Soil structure refers to the aggregates of the sand, silt, and clay particles. Soil structure can be granular, blocky, platy, prismatic, or columnar. Soil structure plays a large role in water retention, and ultimately, productivity.